As noted in the documentation for ordering, it is legal to include multi-valued fields because there are some situations where it returns sensible results, so prohibiting them always leads to undesired restrictions. That does not mean that including them is a sensible thing to do and you should always never do it.

This isn't a bug in Django because the functionality is needed in some cases. The solution is not to include that field in your default ordering (ordering by multiple valued results hardly ever makes sense).